Conventionally, a copper-based sliding material for use in a sliding bearing for an internal combustion engine has been generally manufactured by a continuous sintering process. In the continuous sintering process, a Cu alloy powder is continuously scattered onto a steel strip and then sintered and rolled in series. The copper-based sliding material for a sliding bearing has been required to be free of lead (Pb) in order to meet recent environmental restrictions, and thus a sintered Cu alloy containing bismuth (Bi) in place of Pb has been proposed (see, for example, JP-B2-3421724, JP-A-2005-200703, JP-A-04-28836 and JP-A-05-263166).
A crankshaft of an internal combustion engine tends to be rotated at a higher speed. Accordingly, a sliding bearing has been required to have higher seizure resistance. When the above sintered Cu alloy containing Bi is used as a copper-based sliding material for a sliding bearing, the sintered Cu alloy desirably contains Bi in an amount of not less than 10 mass % in order to obtain the high seizure resistance.
Furthermore, an internal combustion engine has been recently made lightweight. Thus, the weights of an engine block and a connecting rod have been reduced and the rigidity of a bearing housing for supporting a cylindrical sliding bearing has been lowered. Such a bearing housing elastically deforms during operation of the internal combustion engine. The sliding bearing fixed in the bearing housing is subjected to a dynamic load vertically applied to the sliding surface of the sliding bearing from a crankshaft. In addition, tensile and compressive stress are repeatedly applied to the bearing in a circumferential direction. For the reason, the sliding bearing is necessary to have high strength in the circumferential direction as well.
JP-B2-3421724 and JP-A-2005-200703 disclose that a Cu alloy containing Bi is sintered in a continuous sintering process. However, it greatly depends on the Bi content whether the sintered Cu alloy has high strength or not. More specifically, as shown in FIGS. 7A and A′, when a Cu alloy powder 4 was scattered onto a steel strip, many spaces are present in the Cu alloy powder layer 2. When a temperature is raised in a primary sintering step thereafter, Bi melts into a liquid phase 3′ at about 270° C., and then flows out from the Cu alloy powder particles 4 into the spaces between the powder particles 4, as shown in FIGS. 7B and B′. At this time, the Cu alloy powder particles 4 are not sufficiently sintered, and they are not sufficiently bonded. Therefore, Bi in the spaces between the Cu powder particles 4 spreads along the surfaces of the Cu alloy powder particles 4, as shown in FIG. 7C. As a result, the Bi phase 3 in the Cu alloy layer 2 becomes coarse, as shown in FIG. 6. This is significant in the case where Bi is contained in the Cu alloy layer 2 in an amount of not less than 10 mass %. Since the Bi phase 3 rarely solid-solute in the Cu phase, it presents by itself in the Cu alloy layer 2. Furthermore, the Bi phase 3 has strength significantly lower than that of the Cu phase. Since a dynamic load is applied on the bearing, a crack is likely to be developed from the coarse Bi phase 3 or the grain boundary between the Bi phase 3 and the Cu phase, possibly resulting in fatigue breakdown of the Cu alloy layer 2.
On the other hand, JP-A-04-28836 describes that a copper-based sliding material having fine Bi phase can be obtained when a Bi-containing Cu alloy powder is produced by a mechanical alloying process and sintered at a relatively low temperature (400 to 800° C., more preferably, 400 to 700° C.) However, when sintering is performed at a temperature of not higher than 800° C. in a continuous sintering process, a steel back-metal and a Cu alloy layer cannot be sufficiently bonded. As a result, fatigue resistance decreases. On the other hand, when sintering is performed at a temperature above 800° C., the Cu alloy powder is excessively sintered although the Cu alloy layer and the steel back-metal are sufficiently bonded. Thus, the Bi phase in the Cu alloy layer becomes coarse, as described in JP-A-05-263166,